Add rust_build_tests to pdfium_all when enable_rust=true

This will make any bots that enable Rust also actually verify that
building Rust targets is working.

Bug: chromium:1494118
Change-Id: Ic939c8a5d86d10410a3c7b4e268026736a5c0c39
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/113390
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 119c832..42a2c73 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -584,6 +584,11 @@
   if (pdf_is_standalone) {
     deps += [ "testing/tools:test_runner_py" ]
   }
+  if (enable_rust) {
+    # Until there's actual Rust code to use, include the rust_build_tests in the
+    # all group to verify the language keeps working here.
+    deps += [ ":rust_build_tests" ]
+  }
 }
 
 # Makes additional targets reachable only for "gn check". These are not always